How generous! You have a sweet friend!
It depends on what type of diving you will be doing and if it makes sense to dive doubles. If you are beach diving with buddies carrying only 1 cylinder, it may not be worth it. If not, it is very easy to remove the tank bands and manifold. Depending on the set up, you may get two functioning tanks after you buy a couple of tank plugs.
Regardless, I would definitely wait at least 20 dives (minimum) before you try diving doubles. You should have a comfortable foundation before hand. Definitely seek training from someone who is experienced with doubles. Your friend may be a good start but I would see a tec instructor. Take it in confined water to familiarize yourself with it and the necessary gas shut down drills with manifold operation.