ex6150 can I connect two range extenders

can you? yes.

Should you? maybe not. 

extenders take a 50% throughput hit just based off how they work (have to send and recieve with the same chip and can't do both at once). 

meaning the 1st extender is running at 50% of the speed of whatever its getting. The next extender takes another 50% off that so is running at 25% of the router. And thats best case. Because they're also going to loose speed based on distance and obstructions. 

so if you're doing it for speed, it probably isn't a wise choice. 

If you're doing it just to cover a dead spot for a low bandwidth device, then you might be fine

I usually recommend to people needing more than 1 extender to look into moving to a triband mesh system.