Hmm..expecting this... plently of 3gs's on ebay wiht accessories for 350ish +.... so figure two at that price
700 - two 4gs at 199 - the etf for both lines 350 (175 x 2)
so i guess im down $50 but its better then just using profit from selling mine only and having to buy two 4gs at thta point..
Your calculations are way off; you will not be making money on this transaction. First of all, the $50 you cite yourself being down is on top of what it would cost you to upgrade directly, not instead of it. Secondly, you are forgetting taxes and activation fees, both of which will be doubled in your scenario, as well as the three weeks of extra service you will have to maintain on your old lines after opening your new ones (since if you cancel before 30 days you will be required to return your 3GSes). By my calculations, you will be losing quite a bit on this transaction over just selling your phone and using it to pay the upgrade.
Simply selling your phone and using the money to upgrade in June, you and your wife will will pay:
2 x $199 4G
2 x $36 activation
$24 Tax (estimate)
---------
$494 Total
You will take in $400 from the sale of your 3GS, making the total out-of-pocket cost of your upgrade $94 out-of-pocket.
Using your method, in addition to the expenses listed above, you will have the following costs
2 x $175 ETF
2 x $199 3GS
2 x $36 activation (you will have to activate all four lines)
$24 Tax on 3GSes (estimate)
2 x $60 (service on your old lines, which you must keep active for 30 days after upgrade, 3/4 of which is wasted)
------
$964 Total
In order for what you want to do to make financial sense (that is, it will at least not increase your total out-of-pocket expenses), you must be able to cover all of this additional expense through the sale of your two new 3GSes.
This means that you will have to net at least $482 on each of those 3GS phones. If you sell your phones for $700 you will actually be paying an additional $264 out-of-pocket for these upgrades.