That is the challenge... it's a balancing act of staying close enough to your partner for backup and far enough that you can both seek out a monster when they are spread out. We had a couple close calls. I killed one around 96 that was right at the last second.
One thing I do if things seem to be thinning is keep a shuffler alive that's close to me (as long as they aren't spitters they present very little risk) while looking for groups farther away. If things get dicey I just pump half a clip into it, buy myself some more time.
Do not get too focused on mini bosses... do some damage to them but mix in the easy zed kills to keep the combo going.
Do not get locked into a long melee sequence when combo time is in danger. Especially chasing them down with two-three melee hits... just be smart and let it go, if it's not fast, just pump some bullets into them, and move on to your next combo kill.
Get time and herbs early, and have your partner do this as well, right at the start before you kill anything, get all the time. You can have 7+ mins on the clock in 30 seconds if you work together.
Know where to pick up first aid sprays. They are pretty important if you get caught in a nasty loop of taking damage (damn magnum enemies) and they are a fast way to fully heal both players.
Learn to counter the bloodshots. It's easy, an instant kill, and nets you ten seconds each.
Learn to time/counter melee weapon enemies. Don't wait around for the counter, but you can get a feel for timing your approach with their attack pattern. Again, this is instakill (usually) and 10+ seconds. If the timing is off and it seems like you'll have to wait too long, quick shot into melee is a good option: instakill and 5+ second.
As far as high scoring... I think this really comes down to limit breaker and, just as, if not more important: managing the combo bonuses. You need to know where both are, and when/how to use them. We weren't great in this area, and is certainly the difference why we were 3000's on the leaderboards instead of 300's.
Good luck.
edit: Hey and if your partner has a mic, it's really helpful to let the other player know if you're getting in combo trouble. If they have a bunch around them, there's no reason for you to be in panic mode. And vice versa. If you're both in trouble... run in opposite directions and get ready to pump bullets into something (with reckless abandon) probably at range.
