Are you thinking of mixing the two types in one pot, or overcoating one with the other?
According to the datasheets, the basic chemistry is identical, the recommended thinners are the same, nozzles and pressure settings the same, so the chances are the two products can be mixed without issues. Having said that, if the items being covered by this stuff are going to be certified in any way, and the coatings form part of the job specification, it would be best to stick to one product.
There's more VOCs per weight of the 90 hardener and the mixing ratio is 6:1 v 3.5:1 so which mixing ratio would you use? If it's something important that you are painting just get a new tin of 87 hardener, if not critical probably not an issue. If it goes pear shape how much effort is it to fix is how I'd weigh things up.