Can I install two graphics cards on MAC Pro 2008

Yes. Just not the slot 1 (the double-width slot) and slot 2 are 16x and Gen 2. Slots 3 and 4 are 4x Gen 1.1 so not that good for graphics cards.

Also, note that same graphics cards require one or even 2 AUX power connectors but the mac only has two available

There are two main issues, using the right slots in your Mac Pro and whether the Mac Pro can provide enough power for two cards.

As Illaass says slots 1 and 2 are the faster 16x slots and the ones you should use for video cards, if you try using two double-width video cards then you will lose the ability to use slot 3. A video card might need varying amounts of power depending on its needs, the Mac Pro can provide power in the following form as standard.

2 x 6pin PCIe power lead (also known as auxiliary power connector) each can provide 75watts

1 x PCIe slot based power also providing 75watts

Therefore the official maximum power available for a single video card is 75w + 75w + 75w = 225w

For two video cards you will be using two slots so the total available for both video cards is 75w + 75w + 75w + 75w = 300w however it depends on the video cards whether this will be possible in reality.

You can do two cards which only need one 6pin power lead each for example the AMD Radeon HD 7950 which would be 75w + 75w for the first card and the same for the second card. You can do an Nvidia GTX-680 which needs two 6pin power leads and would therefore need 75w + 75w + 75w as the first card and you can do an Nvidia GT-120 card as the second card which does not need any auxiliary power and therefore only needs 75w. You cannot for example do a GTX-680 and a AMD Radeon HD 7950 which would need a total of three 6pin power connectors which the Mac Pro does not have. Similarly you cannot do two GTX-680 cards which would need four 6pin power leads.

Note: Some video cards need 8pin power leads which would supply 150w each, for example the Nvidia GTX-980Ti needs a 6pin plus an 8pin cable along with the PCIe slot power meaning a total need of 75w + 150w + 75w = 300w. So even one of these cards would therefore exceed the official Mac Pro power limits and would even by itself need an additional power supply.

In the cases where the Mac Pro itself cannot provide enough power for two video cards you need to add a second power supply. This could be an external - outside the Mac Pro power supply or it could be one squeezed in to the optical drive bay. See this article and the two links on it - http://www.macvidcards.com/do-i-need-an-additional-power-supply.html

It should be noted that Macs do not support AMDs CrossFire system for linking two video cards, nor do Macs support Nvidia's equivalent which is called SLI. Therefore the main benefit of two video cards is being able to connect additional monitors. As a single video card can depending on the model support several monitors already using two cards may be less useful than you are thinking.