You are allowed to have multiple payment gateways and merchant accounts, but under the new card scheme regulations you must make it easy to cancel subscriptions. As long as your old customers can still manage (i.e. cancel if they want to or aren't getting what they paid for) their subscriptions, you're good from a card scheme compliance perspective.
Details here:
https://usa.review.visa.com/content...cuments/visa-new-subscription-rules-flier.pdf
Double check the 2CheckOut Terms and Conditions to see if they have any specifics in there, though.