Formally there are no restrictions on the joint residence of foreigners and Tunisians . I think the hotel will offer you to take the second single for the full price at the hotel's rates (not a travel agency), but you can live together in your number, to check at night you will not be anyone . I would advise you to apply right now to the travel agency where you paid for your tour, and to change accommodation to double with a surcharge, so it will be cheaper . Or, upon your arrival, contact your representative in the hotel, I'm sure the tour operator will not refuse the extra the tourist again with an additional payment . How much will it be necessary to pay, I can not say for sure . Now, for example, the price for mid-March in your hotel is about 40 EUR per day for half board .