No, you absolutely cannot connect two RF sources to one tuner, because they will not be isolated from one another. This will result in the cable signal being broadcast through your rooftop antenna, interfering with other antennas/receivers nearby. Nor will this function the way you wish, because the cable frequencies are “mid-band cable” which are between the broadcast bands for VHF and UHF. Likely your tuner can tune either cable frequencies or UHF depending upon the menu setting, but not both at the same setting. So you would have to change the menu setting, then re-scan the channels, save the channel list, then tune the channel.
If you want to tune two different sources, you need two tuners. The second tuner can be in a VCR, a DVD recorder, a cable box, or a satellite/DSS receiver, or it could be a standalone settop box. Any of these you buy new today MUST support ATSC (digital) channels by law, but if you need it to tune cable, you must look for QAM tuning capability – and even then only a cable box from your local cable company (or a cablecard) can tune the Premium scrambled channels. To switch to the second tuner, you would switch to the A/V input it was connected to.