It is very unlikely you won't produce enough, but you must resist the urge to give a top up if you want to feed directly.
Your boobs are constantly making milk, so they are never 'empty'. The best way I've heard milk production/supply is as a stream/river:
Your boobs produce milk like a stream flowing into a reservoir (your boobs). Some reservoirs are big and some are small. When the reservoir is low/empty, a signal is sent to increase the flow rate of the stream, filling the reservoir again more quickly. If the reservoir stays full for a long time, the flow rate is decreased and the reservoir will refill more slowly. The stream keeps flowing though, so even if the reservoir is empty, the stream will flow through it.
While the baby is drinking the reservoir of milk, he can drink at the pace he chooses and so seems very happy (oversupply/forceful let down aside). Once the reservoir is empty, he is constrained to drinking at the speed the stream can supply milk. That might be slower than he'd like, leading to frustration. But if you can keep the reservoirs as empty as possible for as long as possible (i.e. feed regularly, without waiting for boobs to refil), then you'll increase the flow rate so that the reservoir fills more quickly in the future and the stream flows a little faster at the end if the feed too.
So basically, feed feed and feed some more :) If you are still worried that you're not making enough milk and can't get a baby to feed, you could express too (pumping even if you're not getting much will help stimulate that stream flow rate too), but remember to give your nipples a break and not to create a rediculous amount of milk!
Feeding twins, particularly if you feed them one at a time, takes a very long time in the early days. It does get much faster and you'll get more time to do non-feeding activities then :) You're doing an amazing thing for your babies and I hope it continues to go well for you :)