Can you power LED strips from both ends?
Your plan is workable.  However, it relies too much on the internal "power bus" running the length of the strip.  The absolute max for using the internal power bus is 10 metres, but that's a bad idea because you will see quite noticeable dimming, and if the strips are not made perfect, they will fail.  What's more, if one of your power supplies fails, you will be driving 5 strips (25m) of load through that internal power bus, and that will fry them for sure. 

I recommend you run "feeder wire" and tie it into the strip buses as many places as possible.  The more places, the less dimming; and yes, it's totally fine to feed the strip from many places. 

I take it these are monochrome LEDs and you aren't controlling or dimming them.  In that case, yes, you can 2-wire power them, but it's even more important you use feeder, as the power-bus traces on 5050 strips are even smaller, particularly the common.  If you wanted to dim/color-control them, you'd need amplifiers and would want to segment it so 2 amplifiers are not in parallel. 

There is nothing wrong with having two DC power supplies in parallel feeding the bus.  DC can be inter-mixed like this, provided they are the same voltage (otherwise the higher voltage supply will do most of the work).  If the DC is "dirty" the power supplies might interact, but that can be solved by putting a diode between each supply and LEDs. 

You are right to use 2x500W power supplies; those cheapie "fell off a truck in Shenzhen" products such as you linked should be derated at least 2x.   With 60-per-metre LED strips I assume 400mA per channel per metre, so 1.2A per metre for 5050s, and 6A per 5m.  

Another way to deal with the voltage drop is use 24V (yes you read right) strips. They are harder to find and more expensive.