unless your decks are really crappy (like beltdrive's) the record will stay in line with the live record for a few seconds, the better your beatmatching, the longer the time it will stay (all logicall i guess). I think every dj checks the phones once every ten seconds when mixing (just hang it around your neck and check it once in a while while mixing in). When mixed in, your hear both records, so you don't need your headphones to hear if the record will go out of line, when mixing out, i usually change the phones to the record i'm mixing out, and check it every now and then during the mixing out.
The better your beatmatching, the longer it will stay in line.

I hope this answers your question

I don't really understand what the question is
Are you having problems selecting a cue point, or keeping the tunes in sync?

You should listen in 1 ear of the headphones whilst mixing!! This obviously helps if you have SONY-MDR-700

the correct way is the way you are most comfortable. eveyone has their own method. you just need to figure out what is most comfortable for you. for me, i cue both songs on the headphones and thats how i beatmatch. but thats just me. im not gonna say if its right or wrong but all i can say is that thats the way that allows me to make the best mixes within my ability.