Its a little bit more than this, although it helps when you are first working out how to double drop.
First of all you need to work out when the drops coincide. You can do this by counting how many bars the drops of your tunes are, and using this to work out when to introduce your next tune so the drops drop at the same time. You can use the dark rings of the vinyl to guess this, but it doesn't work all the time, the only sure way is to count how many bars long the tune is before the drop.
The other thing is you need to have two tunes that are in key. You can double drop any two tracks but if they are not in key, its gonna sound like shit. Only two tracks in key will sound good. Also the main components of each track need to go well together. A 3/4 track will not double drop easily with a 4/4 track, for example.
When you have the tracks set up and ready to drop, you need to eq correct or else you cannot hear anything properly and the two bassline will fight against each other and push the mixer into red. Usually I let the tune coming in to have dominance over the bassline, and the tune coming out I turn down the gain and the low freq a notch and turn the high freq up a tiny bit. This stops the two basslines from distorting each other, but still allows the main components from both tunes to be heard in the mix.
Kinda like lego. If it dont fit, line it up and try again.
Keep trying different tunes with the same track, it outta get easier to hear a key match. Hopefully you got a clean pair of undies cause when you hearyour double drop work out, you might get a bit excited.
I always find double drops by accident! i find the best thing to do is to mix through ALL of your tunes on a regular basis, you'll find the ones that key match and go back and mix them all the time anyway. gradually you'll know by second nature where the drops are and'll be able to cue the tune up in the right place without even realising you've done it! I don't know if this rings true for other people but it does for me.
Recently i've been finding that if i hear a mix with too many double drops in i get bored. it's a great showcase of your technical ability but DJing aint a competition so just tuck a DD in every now and then to get the party jumpin!
practice practice n practice. When you find 2 gud tunes that work write them on a piece of paper or keep them together in your record collection.... i use a piece of paper personally. Remember if the tunes are are slippin out of tune at the last minute its better to take one out at the drop to avoid a mess of battling drum beats, if you fuck it up it jus sounds nasty. also dont have both bass's runnin together cut one out a bit.
Quite a while a go i used to start my tapes that i mixed for my mates with shy fx-the message and shy's chopper remix.It was'nt too difficult as both tunes dropped at the same time it was just keeping them together until they both dropped and when they did....... evertime was wicked,one time even me old man said that they went really well together
I dont think i'd be that good at DDing now as i dont buy todays vinyl but those 2 tunes were wicked for DDing.