If I have a head unit that puts out 50x4 watt. And I have just two speakers connected, does this mean that they can put 100watt x2?? or is it the same 50watt regadless?

I'm not aware of any headunits that allow you to bridge it's amp channels in the same way that a dedicated amp does, so my guess would be no, it's 50W (peak!) per channel and that's as good as it gets :cool:

In reality your head deck is never going to put out 50 x 4 RMS, the laws of physics and amplification would have your head deck running hot enough to fry and egg if that was the case. So your prob looking at something like 10 - 15W RMS per channel (depending on what head deck) and not being amp your head deck is unlikely to double that power when bridged. Not all head decks can be setup bridged either, so just be careful that yours is OK with it.

The speaker outputs on head units are typically already bridged to reach the maximum sustained power output of around 22WRMS. This is the limit that can be achieved without use of voltage steppers. Max of 50, 55, 60W as advertised are only available in very short bursts, in other words hardly ever.