While I do not or ever have had FIOS, no.
You can however, convert the second router to act as if it was only a networking hub/switch/WAP. See REFs
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Only one router can be connected to the ONT.
That said, you can expand wired and wireless coverage by a variety of methods. If you describe the problem you're trying to solve, the model of router you have and what connections (if any) are present between the locations, I or others here can most likely give you some specific advice.