This is my favorite mash-up of all time, made from the songs Radioactive by Imagine Dragons and My Songs Know 
What You Did in the Dark by Fall Out Boy, mashed by AirGirlPhantom on YouTube, link here:
https://www.youtube.com/watch?v=eR99ubyBCGE. 
Sorry about the capo, but trust me, you don't want to play the chords I came up with without it. Also, for the
most part, Fall Out Boy is on the left, Imagine Dragons is on the right.

Tempo: 140

Chords Used: Am, C, G, D, Dm

*Capo on 4th fret*
                                                      Am C G D   Am C G D
                                                    / Wooooooah, Wooooooah
                                                      Am C G D   Am 
Oh, whoa, oh, oh, whoa, oh, oh, whoa, oh, oh, whoa. / Wooooooah, Wooooooah

Oh, whoa, oh, oh, whoa, oh, oh, whoa, oh, oh, whoa.

         Am                    C
Be careful making wishes in the dark, dark
 G                                  Dm
Can't be sure when they've hit their mark
      Am                      C
And besides in the mean, mean time
            G                      Dm
I'm just dreaming of tearing you apart
           Am                 C
I'm in the de-details with the devil
             G                         Dm
So now the world can never get me on my level
         Am
I just gotta get you off the cage
 C
I'm a young lover's rage
  G                      Dm
Gonna need a spark to ignite

          Am 
My songs know what you did in the dark

                                     Am
                        / I'm waking up
                                    
So light 'em up, up, up / I feel it 
                          C
Light 'em up, up, up    / in my bones
                            G                 
Light 'em up, up, up    / Enough to make my 
                             D
I'm on fire             / systems grow

                           Am
So light 'em up, up, up / Welcome to the new age
                          C
Light 'em up, up, up    / To the new age
                           G
Light 'em up, up, up    / Welcome to the new age
                          D
I'm on fire             / To the new age

 Am    C        G            D
Woah, Woah, I'm radioactive, radioactive

                        In the dark, dark
 Am    C        G            D
Woah, Woah, I'm radioactive, radioactive

                        In the dark, dark

          Am                          C
All the writers keep writing what they write
            G                     Dm
Somewhere another pretty vein just dies
              Am                        C
I've got the scars from tomorrow and I wish you could see
                 G                                 Dm
That you’re the antidote to everything except for me, me
          Am                       C
A constellation of tears on your lashes
     G                                 Dm
Burn everything you love, then burn the ashes
Am                        C
In the end everything collides
              G                                  Dm
My childhood spat back out the monster that you see

          Am
My songs know what you did in the dark

                                     Am
                        / I'm waking up
                                    
So light 'em up, up, up / I feel it 
                          C
Light 'em up, up, up    / in my bones
                            G                 
Light 'em up, up, up    / Enough to make my 
                             D
I'm on fire             / systems grow

                           Am
So light 'em up, up, up / Welcome to the new age
                          C
Light 'em up, up, up    / To the new age
                           G
Light 'em up, up, up    / Welcome to the new age
                          D
I'm on fire             / To the new age

 Am    C        G            D
Woah, Woah, I'm radioactive, radioactive

                        In the dark, dark
 Am    C        G            D
Woah, Woah, I'm radioactive, radioactive

                        In the dark, dark

                                         Am          C    G          D
                                       / All systems go, sun hasn't died
                                         Am          C      G             D
My songs know what you did in the dark / Deep in my bones, straight from inside
          Am
My songs know what you did in the dark

                                     Am
                        / I'm waking up
                                    
So light 'em up, up, up / I feel it 
                          C
Light 'em up, up, up    / in my bones
                            G                 
Light 'em up, up, up    / Enough to make my 
                             D
I'm on fire             / systems grow

                           Am
So light 'em up, up, up / Welcome to the new age
                          C
Light 'em up, up, up    / To the new age
                           G
Light 'em up, up, up    / Welcome to the new age
                          D
I'm on fire             / To the new age

 Am    C        G            D
Woah, Woah, I'm radioactive, radioactive

                        In the dark, dark
 Am    C        G            D
Woah, Woah, I'm radioactive, radioactive

                        In the dark, dark