Stacking Caster Bonus

Is the Wand Wrapping of Memory and a Forget Me not Inscription stackable to get 40% chance to half skill recharge? Could I get it to 80% with two mods on both a wand and an offhand?

They do stack but it adds up to a 36% chance of halving. A wand + offhand with those mods for both casting and spell recharge is called a 40/40 set.

The bonuses stack, but the stacking is multiplicative, not additive. When using a wand and a focus that each have a 20% HSR mod, the combined chance of HSR is 36%, not 40%. We refer to a set with both HSR and HCT as a 40/40 set for convenience rather than using the messy actual numbers.

To put two mods on a wand, you need a wrapping and an inscription. But there is no inscription for a wand that gives HSR, so there is no way to put two HSR mods on a wand. To put two mods on a focus, you need an inscription and a focus core. But there is no focus core that gives HSR, so there is no way to put two HSR mods on a focus.

The best you can do is one HSR mod on each. However, if you could put two on each, the total chance of HSR would be 59%, not 80%.