Linking power banks and ports

Imagine a situation when you need 50,000 mAh, but you only have 2x 30,000 mAh power banks (PB), to power a single device.
One way to handle that, is by connecting your device to PB-1 while it’s being charged by PB-2.
That works, but I think at the cost of extra conversion losses.

Can I safely use an Y-cable to draw juice from both PBs at the same time to power the device?
I think I safely can, but there may be certain (smaller) things I overlook.

If so the capacity doubled, without needless conversion losses.
Because the load per PB is lower the efficiency of both PB’s goes up.
Assuming the Y-cable can handle the load, the maximum amps that can be drawn doubles too.

That brings me to my last question. Yesterday I was reading this review and the major drawback of that powerbank is the max output per port is only 2A. I often need a little extra.
Can I safely link 2 ports on the same PB with an Y-cable to boost the maximum output?

Yes.

Good cables are these:
delock.de/produkte/G_763_Y-Strom—und-Datenkabel.html?setLanguage=en
startech.com/Cables/USB-2.0/Micro/3foot-USB-Y-Cable-for-External-Hard-Drive-Dual-USB-A-to-Micro-B~USB2HAUBY3