Straight off the top, if you go to Tmobile's new uncarrier plan, you're bill for both of you with unlimited calls/texts and 500MB of data apiece would be $100. Not a bad deal.
Net10 (Tmo version) is the cheapest for you guys together, $90 for unlimited calls/texts and 2 GBs of data. However, Net10 has crummy customer service, Audio and Video streaming violate their ToS (so no Pandora/Youtube without risking them canceling you). They do have voice roaming.
Solavei would be $98, also on the Tmo network. Unlimited calls/texts + 4GB of data @ 4G speeds (throttled beyond that). They roam voice/text/and up to 100MB of data). Their customer service seems to be lacking, its a relatively new company that has grown fast over the past year. They do have a very helpful set of forums though. I use them and am happy with them... if you're interested in them I may be able to get you free SIM cards (depends on which iPhone you've got). They have a referral program where if you refer 3 people you get $20 off your bill a month... which is great except some people treat it as a MLM/pyramid scheme and that may leave a bad taste in your mouth. If so, avoid them imo. Service is solid, but if that sort of thing irks you too much, you'll never get past it. Bothers me a tad, but not too much really.
Straight Talk had launched a bring your own verizon phone program last week, but then they pulled it back, not sure what the deal is/was, or if iPhones would've been eligible. Can't tell now since they shut it down apparently. Weird. They're $90 for both on Tmobile, but their cousin's with Net10 and have the same customer service issues.
Now, if you get seperate accounts instead of a family plan, there are cheaper options still. All on Tmobile.
Platinum Tel (PTEL) Mobile: $40 = Unlimited talk/text + 250 MBs of 4g speed data, throttled beyond that limit
Simple Mobile: $40 = Unlimited talk/text + 250 MBs of 4g speed data, cut off at the cap
Spot Mobile: $39.99 Unlimited talk/text + 500 MBs of 4g speed data, throttled beyond that limit
All 3 on Tmobile. PTEL and Simple do not have roaming. You step outside of Tmo's footprint, no service. Spot Mobile does, but they charge you for it. Roaming charges can be a bit painful too. I don't know much about PTEL or Spot, but I do know Simple supposedly has poor customer service too.
So, anyways, all of these options involve going to Tmobile's network, which would mean buying new phones. Something to keep in mind. When is your contract up?