NASA certainly wouldn’t have done that anyway, out of concern for flame or blast or sound-pressure damage from one booster damaging the other.
The Skylab I Saturn V was on the pad at the same time as the Skylab II Saturn IB, and there were several times that both pads were occupied by Shuttles.
Correct; here is a photo of two shuttles on the pad simultaneously. STS-125 (Hubble servicing mission) launched while STS-400 was on the pad standing by for a possible rescue mission: https://upload.wikimedia.org/wikipedia/commons/5/53/Space_shuttles_Atlantis_(STS-125)_and_Endeavour_(STS-400)_on_launch_pads.jpg
The Saturn V and Saturn IB on pad at same time for Skylab: https://images.app.goo.gl/TxQw3uqfrLVtHby2A
Two Russian N1 launchers were on the pad at the same time. The N1 was roughly equivalent to the Saturn V in some metrics: https://images.app.goo.gl/KXygzindEwSWRkCX7
The original Saturn/Apollo plan envisioned a higher flight rate and concurrent pad operations with two Saturn V vehicles, so the infrastructure was built to support this. That rate did not materialize but the VAB was capable of supporting assembly of three Saturn Vs at the same time, and this was actually done during 1969: Vehicle Assembly Building (VAB)