v.90 modems are limited to 33.6kbps upload speeds, thus 2 56K ANALOG v.90 modems can never connect at higher than 33.6kbps.
ISPs use different modems in their modem banks that can send and receive at up to 56k (fcc limits this to 53k legally through power output restrictions)
If you can test this, I have a suggestion. The new v.92 modems increase the upload speed to 48kbps. Maybe 2 v.92 modems could connect higher than 33.6k, depending on line conditions and the number of times the signal is converted from analog to digital and back at the telco's switches.
Then again, the new, higher speed may be limited to dial-up access to a digital modem bank at an ISP.