#1
so my band has written a song that sounds like two songs. it starts off sounding like a slow mellow rock song and then gets really really fast, then slows back down. the problem is when it goes from slow to fast the first time it sounds alot like 2 diffrent songs. we already have a rough structure to the song and have the lyrics written. any ideas on how to make it sound like one complete song?
#2
Sometimes you have to look at the song, and think is this going to work? If it sounds really different, then you may have to rethink the song. A lot of songs start out slow and then go fast, and there's nothing wrong with that but usually the slow bit is a "softer" version of the fast bit. Usually revolving around similar chords, the initial idea. Then it is developed fully in the fast section. You want to keep some sort of familiar idea.

Things to look out for:

- Is your fast section in a different key? This can confuse people and lose some continuity.

- Is your fast section completely different chord wise, from the intro? Sometimes it's a better idea to use the same chords, or at least some familiar idea or melody

- Is there any real point to the slow-fast contrast? Does it relate to your song?

So yeah, overall nothing wrong with this as long as it has a sense of familiarity. Keeping the fast and slow bits in the same key would be a start. But try and keep some chords and or melody in both parts so you can get some structure.

The best songs using this method introduce a little idea in the intro, and then in the fast section you get to see that idea fully developed. This is what you should aim for - SOME form of continuity.
#3
If the concern is only on the first time, and not the second, write a bridge linking the two parts to help it make sense. The bridge could have lyrics, a break-down, solo, whatever you want - it's purpose is to link the two. It can be awesome when done right.

Gun's and Roses employ this method on a lot of their songs - think about the changes before and after a lot of Slash's solos. Usually Slash doesn't solo over the verse chords, so a bridge is employed to link it to the rest of the songs.
And no, Guitar Hero will not help. Even on expert. Really.
Soundcloud
#4
Either a bridge as stated above or do some guitar screeching sort pinch harmonic effect or whatever, perhaps a very over the top slide, before blasting in the fast bit. SO either bridge or go crazy.