Antenna Proximity and Frontend Overload

As the title suggests, how easy it it to cause front end overload damage?

The Context:

I have a 11m vertical dipole on the same mast as my VHF/UHF Antenna,the boom supplied with the dipole is silly short at around 40cm.
therefore I have these two antennas a school ruler apart (for those of you who have only used computers at school that's 30cm lol)

1. Can the HF transmission overload the front end on the VHF/UHF unit or vice a versa or BOTH.
2. Whats the remedy as I need unfortunately both antennas on this mast, will a longer boom up to a meter really make any difference? Perhaps the use of an antenna switch on the back end of the VHF unit so to disconnect the antenna while transmitting on the other (tbh I've seen inside some of the antenna switches and I've been appalled at the poor construction.

Looking forward to you views, keep it relatively simple, my head hurts with it gets all 'Einstein' on my butt!

I expect you'll get some directivity on both of those. I'd also check the tuning, especially on the VHF antenna.

2m amateur band is very close to a 5th harmonic of the 11m band. I'd maybe throw in a low pass filter on the dipole to be sure.

Mobile I've had my screwdriver antenna not much further away from my 1/4 wave 2m antenna on the car and run 100W on 80-10m and 25W out of the dualbander. Been doing this for over half a decade without issue on either.

Hi Mike.
I expected some directivity on the dipole which is fine as its solely up for local chat, I had not thought about it so much on the co-linear but i guess having the dipole next to it will effect the radiation pattern somewhat, tuning seems fine on 2m, apparently my meter wont read SWR on 70cms !!
Everyone around here mainly uses repeaters, not much simplex use so that should not really be much of an issue.

Ill pop a low pass filter on the HF side, think i have round kicking around somewhere, although ill probably buy a new one as the one i had seemed to be messing with the SWR at the radio ?

Yeah a low pass filter shouldn't alter the SWR so either there's a problem with it or the patch lead. There's very little inside them component wise so it may be worth taking the lid off and having a look to see if there's anything obvious.

Ill do just that AND check those leads