I have 3 lookup fields on a form, first two list two different types of training, and the third hidden one needs to contain both selections from the first two lookups. This is all so I can then use a custom function to show training participants. 

This should happen as lookups 1 and then 2 are selected, so no extra steps are needed.

Lookup 1 dropdown select - Training type 1

Lookup 2 dropdown select - Training type 2

Lookup 3 multi-select auto adds - Training type 1, Training type 2

Thanks