Question about Digital voucher for CHF 10.–

Also not sure why you need to to use a voucher to buy another voucher, but you can use portions on a voucher.
I had a couple of 10CHF vouchers (let's say 2x10CHF) and I got something worth 19CHF. One voucher had been fully used, and I got 1CHF left on the last voucher. I can then use that remainder on another purchase.
Hope this helps.

I am not sure I understand the question but why don’t you just buy 2 x 10- voucher then and redeem them. Note you can purchase and redeem as many vouchers as you like