With two Enduring Scalelords, is it possible to create an infinite loop by putting a +1/+1 counter on a creature, then because of that, place a +1/+1 counter on both Enduring Scalelords, and because of those gaining +1/+1 counters, they get a +1/+1 counter for the other Enduring Scalelord?
To quote directly from the rulings on Gatherer:
If you control two Enduring Scalelords, putting a +1/+1 counter on one of them will cause the ability of the other one to trigger. When this ability resolves, you’ll put a +1/+1 counter on the other Scalelord. This will cause the ability of the first one to trigger. This loop will repeat until you choose not to put a +1/+1 counter on one of the Enduring Scalelords.
Yes, that would create an indefinite loop. You have to choose a number for how many times the loop is to be performed.
In the Comprehensive Rules, this situation is covered by section 719, Taking Shortcuts:
719.1b Occasionally the game gets into a state in which a set of actions could be repeated indefinitely (thus creating a “loop”). In that case, the shortcut rules can be used to determine how many times those actions are repeated without having to actually perform them, and how the loop is broken.
You tell your opponent how often the loop should happen, and if your opponent does not propose a different series of events, your Scalelords will get that many counters.
You have to choose such a number, because there is no such thing as "infinite" in the game. Every number is a definite one. The game is also not a draw, because not all steps in the loop are mandatory:
719.4. If a loop contains only mandatory actions, the game is a draw. (See rules 104.4b and 104.4f.)
104.4b If a game that’s not using the limited range of influence option (including a two-player game) somehow enters a “loop” of mandatory actions, repeating a sequence of events with no way to stop, the game is a draw. Loops that contain an optional action don’t result in a draw.