You probably noticed in Step 10, the gantry sides are pretty wide. It's wide so the gantry sides can position the router at the center of balance. Even though it eye-balled, it's probably pretty close. The wider the gantry side, the longer the linear slide bearings can be thereby giving the gantry more stability. No need for double rails, just deepen the gantry. One caveat, the larger the cutting surface area, the more flex you will get, so if you're lookin' to make an extremely large table, the wood will need to be framed in a manner that flex is prohibitive.

Ok, enough of that, let's talk about the video. I'm making the linear slide bearings, an extra long pair. The length matches the width of the gantry side. You already know how to make these things, but I'm showing the video to provide a log of every action I have taken. The video is an abridged version of Step 1, but it shows the extra length of the aluminum angles and the positions of the bearings (at the extremes of the angle). The load is transferred from the gantry to the ring bearings. The farther apart the bearings are, the more stability you will get. Be careful not to make the linear bearings too long that they stick outside the gantry sides, where you may get bending in the aluminum angles (although, the extensions would have to be extreme).