If you have a 2 month Interrail pass both months will be activated when you activate the pass. There is no way to only activate a part of the pass. As an example, if you activate the pass on the 8th of June it will be active until the 7th of August

But I guess-though you are not clear about this_ IF it is a pass like ´use 10 days in 2 monthes´ then of course you can use these 10 days in 20 days-or 28, or 37, etc. When day 10 is used. the pass becomes worthless.