Will returning two 1/2" lines to 3/4" get a volume return?
It depends on your flow rate, and the length of your run. The short answer is you won't have a noticeable pressure drop for a relatively short run.

Using this table, the nominal inside diameters of the pipes actually give you a slightly greater cross sectional area from the two 1/2" pipes (0.608 sq. in. vs 0.533 sq. in.). 

However, the head loss is still greater for the two 1/2" pipes. For the 9 GPM you'll need for the body jets, you'll have a loss of about 15.8 ft/100 ft pipe on the 3/4". For the 4.5 GPM you'll need through each 1/2" pipe, you'll have a loss of 16.9 ft/100 ft pipe.*

This comes out to a loss of 6.85 psi per 100 feet of pipe for the 3/4" pipe, or 7.33 psi per 100 feet of pipe for the two 1/2" pipes. You'll have to add another 15.6' of equivalent footage for the two extra T and elbow fittings as well. 

*Note: I linearly interpolated these values, but the head loss curve is NOT linear. So not an EXACT answer, but probably close enough.