Using CEF balancing, you may also enable per-packet balancing (rather than the default per-destination).
Under each Serial interface you will need to specify the command: ip load-sharing per-packet -- this is as close as you will get to Multilink PPP type distribution while using CEF balancing. Note that this may cause noticable problems for certain applications (VoIP). When such protocols are riding the connection, use per-destination. You will also need to make sure that your ISP has this enabled on each Serial interface on their end as well.
Multilink PPP is another option, but from my experience, Sprint will refuse to run this with you,
but other ISPs might if you were to ditch Sprint at some point. MLPPP is known to be a CPU hog compared to other balancing methods, but at two T1s I'm sure you wouldn't notice.
Another alternative would be to balance using BGP with a different ISP. One of your ISPs will assign you a /24 or larger network. You then have your other provider announce the network for you (they will likely need a letter of authorization from the assigning ISP).
BGP balancing on only two T1s is not a cakewalk and could require frequent tuning. Especially difficult to do if your ISPs are the ones running BGP -- you would need to call them every time you'd want to tweak BGP.
Running BGP on your own router would give you more control along with more administrative overhead, and additional memory requirements (due to the large size of Internet routing tables), which may force you into a bigger router.
I've seen some people run IMA (Inverse-muxed ATM) for balancing as well. The balancing should be great, but the hardware isn't dirt cheap, and you have ATM cell overhead to deal with.
