It should be pointed out that the hardware based timestamping is only needed if you need time alignment better than a half second or so. With USB transfers, various buffers, NTP based alignment of the host computer's time, and some extra code on the host side you could do a coarse time alignment, probably with less than a half second of error.
You could also time align the streams if both radios receive the same signal and you know the distance (and other details depending on the precision needed) between the transmitter and your two receivers. This is done by many protocols like the cellular standards to create a distributed timebase, but quickly becomes non-trivial.
The B200 does timestamping in the FPGA and can use a 1PPS signal to align to GPS time. It is more expensive than the HackRF, but much less than the $3000 mentioned above.