Double dosing Bridge gives you the same dose of Fadogia Agrestis (the test boosting compound) that a single dose of HCGenerate does.
HCGenerate is best used 2 weeks or so into a cycle, as it prevents/slows natural testosterone production shutdown. Bridge is best used between cycles, to help keep that "On-Cycle" feeling, boost test levels, and help retain those hard earned gains that were made.
Obviously, it's more cost effective to run them as listed. In the case that HCGenerate is out of stock, Bridge is well worth the little extra cost to run in it's place.