tbarnstormer: The TM808, just so you know, will only find objects the size of a soda can and larger. And even a soda can sized object, can be a difficult signal (ie.: that's the smallest it can go). Ideally, it was meant for larger objects (lunch box sized and bigger). So it would depend on how tightly packed the coins, in your scenario, are, and how many are there, etc.... When I say "it depends on how tightly packed", that means if the coins are loose, and not touching tightly, the machine has a harder time seeing it as a single object (in much the same scenario of gold bracelets that give a weak signal, because the detector is seeing individual links, if it's not TIGHTLY wadded up)
You could certainly get another standard coin/relic machine if you wanted. But be aware, that if there is a lot of small trash in the search area (tabs, nails, clad, foil, etc...) you will go crazy having to dig out all that. So if you know for a certain fact, that 1) your object is big enough, and 2) you simply DON'T want to dig anything else EXCEPT that/those larger targets, then 3) a 2-box unit is a good choice. Before you dump the 2-box machine, try this: take it out to a clean stretch of sandy beach (or out in a cow pasture far away from any modern influence, junk, etc...) in a stretch that is fairly clean of any human influence. Bury a metal thermos, or tool box, or whatever. Make sure it's a conductive metal (ie.: not iron). Bury it at the depths you suspect, and practice on that target. Practice walking all over, not just on the known spot, so that you will see the difference between silence and the target.
Without knowing what your problems with the TM808 is, it's hard to say if you should move to a different machine or not. I have personally hunted for many people, that prior to calling me out to hunt their property (for grandpa's buried stash, or sister's gold ring, etc...) did try to rent or buy a machine, and look for it on their own. But since they'd never handled a detector before, they simply don't know what they're supposed to be listening for, don't understand the controls, etc.... They usually give up and call out a hobbyists, who can immediately recognize what the person was doing wrong, and end up finding whatever object it is. So if there is a club in your area, you might call on the help of someone proficient in the type hunt your after. If they show up with a standard coin/relic machine, you will have the afore-mentioned problem of them having to weed through all the small targets. It might seem easy to tell them "pass small stuff", but the problem is, if there are belt with continuous "small stuff", something 4 ft. deep can start to be indistinguishable from lots of close-by surface stuff.