That's $75 for both lines on TMo, correct?
The cheapest you'll really be able to go for two GSM prepaid phones is about $20 a month, and it's going to require the two of you to scale back a fair bit on your cell phone usage, which might be difficult to do without a VoIP plan at home which is going to shoot you up to at least $30.
For that same $30 a month, you could go to Ting and get a pool of 1000 minutes, no texts and no data on two lines, effectively saving you $35 a month and not requiring you to modify your usage habits at all... the only difference is you'll have buy-in costs for new equipment. Ting offers the Samsung M370 basic flip feature phone for $77 a pop. By my calculations (so long as you're not under contract), your ROI for the new equipment for the switch from GSM to CDMA with two of those handsets would only be about 4.4 months.
https://ting.com/We've also got a huge resource on communications
here if you'd like to explore a few other options like Platinumtel, Airvoice, H2O Wireless, PagePlus, etc... but for the money spent, that's going to be your biggest bang for buck with the easiest usage transition.