How can I (if it is possible) to set up the network gateway if I have 2 internet lines from 2 different ISPs? Furthermore, one of them is forwarding /28 static ip for (into) our network. Is there a schematic diagram to refer to?
Using two or more ISPs is totally feasible, normal and even encouraged. You can:
How to set it up will depend on what you are using to route the traffic. If it is a linux box you can refer to the Linux Advanced Routing & Traffic Control guide (section 4.2). For other systems you will have to look for some documentation.
I didn't get what you said by schematics, /28 means 16-2 valid ip addresses, but theoretically each ISP must give you at least one valid IP to work with.
You can use two lines, in fact we have two lines here, to two different ISPs over different media.
However, you can NOT bond these disparit networks together (to combine two 10Mb links into a 20Mb link). At our router, we split traffic based on either destination, or if no specific destination, then round robin.
Regarding the /28 - this is no problem because as I mentioned the two links have to stay seperate.