The best way is to first match the BPMs. When this is done the creativity can flow unimpaired. The best way is to increase the lowest tempo to match the highest(compress the lowest track's length). Another way is to adjust both songs to "meet in the middle". Some software's better than others for time stretching. I use Serato software when I play live, and the pitch lock sounds great.
You can find the BPM of a song by using software, or by doing some simple maths, if you know how many bars are in the track, and you know the time scale from first to last beat, then you can find the BPM.
If you still want to match one track to the other by splitting it up, split it into eights notes at least, and to save hastle, don't decrease the tempo on the split track(this will create gaps). Maybe Bitwig has a way of automating this splitting process. Remember to be precice with cutting and placement.
In the past, I would create mash-ups on my decks with just the two vinyls, using the pitch adjustment to match them. No pitch-lock, editing on-the-fly and people loved them. There is no 'wrong' way to do these things, all that matters is if you like the outcome.