Yes, for the two TotalBoat epoxy systems that have variable speed hardeners, you can mix hardeners within the same epoxy system to adjust the cure time. These systems are the TotalBoat High Performance 2:1 Epoxy system (2:1 Slow, 2:1 Medium and 2:1 Fast Hardeners), and the TotalBoat Traditional Epoxy 5:1 Epoxy system (5:1 Slow and 5:1 Fast Hardeners***). Only use TotalBoat 2:1 Hardeners (Fast/Medium/Slow) with TotalBoat High Performance 2:1 Epoxy Resin; only use TotalBoat 5:1 Hardeners (5:1 Fast/Slow) with TotalBoat Traditional 5:1 Epoxy Resin.
For a proper cure, be sure to use the mix ratio (by weight or by volume) indicated on the label. If one of the hardeners has a mix ratio by weight, for instance, of 28A:100B and one hardener is 27A:100B, it's acceptable to combine these hardeners because the mix ratios (by weight) are almost the same.
***IMPORTANT: Do not combine the TotalBoat Tropical Extra Slow Hardener or TotalBoat Crystal Clear Hardener with each other or with the 5:1 Fast/Slow Hardeners to adjust the cure time of the 5:1 Epoxy System.