You put Flooz into an account, like a checking or credit account. Every single site that accepts Flooz right now only lets you pay with ONE account. So theoretically, you've sent two payments of $60 each to two different people, and you received back two payments of $15 each, which goes in a single account.
You end up with three accounts, two of which have $60 Flooz in it and one with $30. You cannot spend all $150 in one transaction. You can only spend what's in each account - so $60, $60 and $30 - or possibly $120 and $30.
It all depends on how you redeemed the Flooz. They send a link in an e-mail message that the recipient clicks on. They have the option of putting the Flooz you sent into an existing account or opening a new one. So - if want to combine the two $60 gifts, you probably could, by redeeming one (creating an account), then redeeming the next one and putting it into the same account. If Flooz catches you trying to put your earned Flooz (the $30) into the same account that the other two went into, they'll take it away, because you're only allowed to collect the free stuff on Flooz you send to someone else.