Masking Quantities

Is there a way to mask a quantity? In my store, I have a product with 21 left in stock, and I have a gift voucher with 21 left in stock for the other product.

Is there a way to make it so that when you buy the product, and not a voucher it puts BOTH quantities down, but when you buy a voucher, it doesn't but the other products quantity down. I don't want people buying more vouchers for a product than I have of that product in stock. Thanks.

Is there a way to make it so that when you buy the product, and not a voucher it puts BOTH quantities down, but when you buy a voucher, it doesn't but the other products quantity down.
I don't know exactly what your voucher is???

But a way to do what you are stating, I think, and easy enough to setup and check

To your product add 3 attributes in drop form
Please choose
Product
Voucher

These will not be set to add any cost to the product
If product is selected product qty is decremented
If voucher is selected product qty is decremented

You order details will specify if the product or the voucher is purchased so process accordingly

You will not have nor need any stock for vouchers if done this way