In some instances, floor joists fail as single members at 16" O.C. Doubling everyone is overkill and 12" O.C. works but is difficult for HVAC and Plumbing Contractors to work with. Noting to double every other joist at 16" O.C. is something our firm specifies on occasion as comparable to 12" spacing. When you evaluate total number of joists over say a 4'-0" span, doubling every other at 16" O.C. would place one joist more than 12" O.C. would provide below that same section of subfloor.
Guest
This could be implemented, but I would just put a note on the results page "12-in OC spacing is equivalent to doubling every other joist @ 16-in OC", which looking at some quick numbers, your method is actually a tighter spacing than 12" OC, 32/3 = 10.67" each joist, so using 12" would be conservative.