Hi,
1. In your case I guess that using the options will be a better solution.
You have product A - 15€, product B - 20€, you will need to create another instance of product B (in another category, a hidden one) set its price to 5€ and add it as option to the product A. This way, only product B cost 20€ but if bought with the product A it cost only 5€ so a total of 20€.
2. You have to set your website currency in LTL in Configuration > Main > Currency, and in the product pages set the product price in EUR.