There should be no concerns charging 2 devices at one time? I know these bikes don't have a lot of spare power. I actually have a spare lead I could setup to run to the rear of the bike.
Also, charging the SENA overnight with the bike off... this won't drain the battery?
People's experience can be a guide for this, but here is a simple calculation to show where charging 2 devices should get you.
Bike battery total energy at full charge, (a 14 Amp*Hour battery at 12.5 V average over the discharge curve at low current) : 14 AmpHour * 3600 seconds/hour * 12.5V = 630 thousand Watt*Seconds (aka Joules)
Cell phone battery energy at full charge, (a mondo 3000 mAH or 3.0 Amp*Hour battery at 3.7 V nominal) : 3 AmpHour * 3600 seconds/hour * 3.7V = 40 thousand Joules
If you had a perfectly efficient charger and the cell phone wasted no power when charging the battery (and if batteries gave back all the energy put into them), then your fully charged battery could charge 15.7 phones with mondo batteries. By the time you account for charger inefficiency, the phone dropping 5V down to 3.7 dissipatively, and batteries taking more energy to charge than they give up upon discharge, you have to adjust that phone count downward by a factor like 0.8 * (3.7/5) * 0.9 = 0.53, you are down to 15.7 * 0.53 = 8.3 phones. (The 0.9 figure for charge/discharge efficiency is conservative for low current discharge, such as when running little chargers.)
Start with a half-charged bike battery because your heated gear keeps the charging system from getting the battery to full charge on a long ride, and you could find that fully charging 4 dead phones overnight would nearly deplete the battery.
Did you say 2 devices? Unless they are much worse power pigs than a modern cell phone / wanna-be computer, you should still have plenty of energy left to get your bike started in the morning.
BTW, your Sena headset has a far smaller battery than a modern cell phone. It is of no consequence in this energy management issue.