Merge it the same way as other things, brace it, and use the interlocking method. Look geomerging in Forging 101 for an indepth method of doing so.
yeah u dont want to force one into the other. just place one. put it late respawn then put the other sign in nearly the same place as the old one.
And if you find the interlocking method annoying the just use doors as if you were merging a box or what not. And no one say "a door is interlocking, it just does it by itself". I know. If you need help message me.
I use teleporter nodes to brace and a single box to push it in. I never could do it using doors to push it in.