First off, from my understanding having 2 Ethernet connection in the back of your server's motherboard are as follows:
- older server motherboard used to come with 100mb line and 1000mb (1 GB) line, so you used the Ethernet port that suited your network setup.
- it had 2 because if 1 failed in the future, you had a second one just in-case, cause most of the time other companies will not have a network adapter that would be usable on that specific motherboard because it's a server motherboards. most network drivers are geared toward normal public consumer use, not servers.
- you can setup 1 Ethernet port for internet and 1 for LAN only, this creates a type of firewall for your network setup.
That being said, the way you have your office network seriously puzzles me, why on earth would you setup 2 different networks in the same office that need to communicate with each other and then order 2 seperate sources of internet.
Let say your office is split into 2 parts, LEFT SIDE and RIGHT SIDE. You use 1 Ethernet connection from your server to 1 Router (wifi 1, Leftside), This should give everyone the same amount of access to the server and your internet connection. Now if the range of Right side does not reach Leftside Router. Then you want to buy a network EXTENDER, this will grab the connection from wifi 1, and boost the signal to RIGHT SIDE allowing all of your employees to stay on the same network. OR buy a more powerful router N450+ and set it up on your drop ceiling in the middle of your office, or something to that effective.
THIS is under the assumption that you have 2 wifi's because you want to spread the wifi throughout the office because certain people can't reach wifi 1. If that isnt the case, your setup is really useless and a waste of resources, you're spending 2x the amount of money on the 2nd connection. If the speeds are not fast enough for 15 users on wifi 1 which is why you have 2 connections, then upgrade your internet speeds from your ISP.
All that being said, you need to hire a REAL IT guy to setup your network properly. And to answer your question of merging the 2 networks together, i honestly doubt it. Your setup would make sense for this scenario (aside from the 2 seperate ISP connections), lets say you own a building with 2 floors, you can setup the entire building on 1 internet connection, but use another router to separate LAN from the first floor and the second floor for security reasons? or i don't know? keep the 2 floors seperate.
Other then that, lose the 2nd internet line, stop using the 2nd Ethernet connection on your server, and use 1 router for everyone.