Can I charge an ebike from another ebike battery?
Yes in theory but the gain is not practical. You’re better off swapping batteries than trying to jury rig something. 

Let’s do the calculations. 

You note that the AC charger puts out 36VDC at 2 amps. That’s 72 watts per hour. 

Even a modest eBike has an 8Ah battery, for a total of 36x8 or 288 watt-hours. 

At 72 watt-hours per hour charging it takes four hours to charge. So you’re trying to fill your “gas tank” with a very tiny spigot, so to speak. 

Your internal batteries BMS charging  may stop taking a charge well below the output of your external pack. My 36v bike for example wants to see 42v on its input. So you’d need a boost circuit. 

Add to this charging inefficiencies which are usually on the order of 85-90% with each step and it’s not worth it. 

So just get another spare pack and exchange when your empty. Or rebuild your current pack so it’s larger but scale the BMS accordingly.