Epoxy systems such as MAS, TotalBoat, and West System generally include hardeners with a few different cure speeds, for example, fast, medium, and slow. Typically, you can combine hardeners with different speeds to customize your cure time, as long as the hardeners are within the same epoxy system. Just be sure to use the mix ratio indicated on the label, and your mixed epoxy will cure properly. It's important to only use hardeners in the same system, that are the same mix ratio. However, if you have one hardener speed that has a mix ratio by weight of, for example, 27A:100B and one hardener that's 28A:100B, they would be okay to combine because the ratios are so close.
By 'epoxy system' we mean brand-specific as well as product-specific. For example, TotalBoat 5:1 Traditional Epoxy has 5:1 Fast and 5:1 Slow Hardeners. TotalBoat High Performance 2:1 Epoxy has its own 2:1 Fast, 2:1 Medium, and 2:1 Slow Hardeners. Only use TotalBoat 5:1 Hardeners with TotalBoat 5:1 Epoxy Resin; only use TotalBoat 2:1 Hardeners with TotalBoat High Performance 2:1 Epoxy Resin. Another example is the MAS LV Epoxy system, which has Fast, Medium and Slow hardeners; these hardeners should be used only with MAS LV epoxy resin. Before you mix hardeners within any epoxy system/brand type, be sure to check the manufacturer's recommendations, for best results.