Short answer: no.

Long answer: It would be exactly same as trying to connect 2 devices (USb mouse + USb keyboard) to one port of desktop computer, without using HUB. Try it, but don't blame us if Your ports refuse to work after due to blowed fuse (ho ever, it should just don't work, anyway, except for really screwed up cables)

Cables you're mentioning are totally for different things - most of them time, it's kind of Y-cable for providing additional 500 mAh of power from spare port - for power-hundry HDD's, or BlueRay writers, or whatever.

Ho ever, *sometimes* You may get Your hands on cable that actually contains very small 2-ports hub bundled inside, so it looks exactly like variant of Y-cable, but work like hub. No problems with using it for HEN, but to be honest, I find small USB 2.0 4 ports hubs much more sane thing (they are really small anyway, and contain 4 ports, not just 2...). Also, with hubs bundled inside cable, You can usually forget about powering them up with external power supply. Unless You modify them further, to also contain another plug, just for powering up