Thats a 50watt output charger. You can do that, Y'ed together series that will be 16.8v and the charger will limit current to stay under its 50watt output limit. 50watts/16.8v = 2.9amps. Charger will only be able to charge at about 3 amps and stay under the 50watt limit.

Given the hassle of balancing them together, I think you'd do it quicker to charge them separately where you can charge at 4amps, but it may be a toss up time wise, Y'ed together versus doing the separate.

What do you plan to do for balancing them?