Sometimes when our customers pay cash, they pay part of the invoice in USD and the other part in Tsh. Would I then have to issue 2 seperate receipts (1 for each currency)?
Yes. A transaction can only be in one currency. In your example, the debit for the transaction goes to the cash account, while the credit goes to Accounts receivable. That cash account can only be denominated in one currency. So you first need two cash accounts in the two currencies, then two receipts.