From what I know of comercial antenna designers and use from antennas mounted above each other vertically or horizontally separated, you cannot achieve at best 20 dB of isolation if you really put a lot of work into optimizing your antenna placement using meassuring equipment for finding the best isolation.
If you really have really only the one transmitt frequency VHF 157.625 MHZ close by you could use a high quality Band Reject-/Band Stop-/Notch-/Stub-Filter. Such a Band Reject-/Band Stop-/Notch-/Stub-Filter can be designed using coax-cable, capacitor/inductors, from PCB. The rejection and bandwidth will vary with the design and quality of the components used.
The disadvantage with all such circuits employing a notchfilter, is that it will also notch multiples of the notch frequency it was designed for e.g. 2 x 157.625 MHZ and so on.
A single series tuned resonant circuit/Trap-Filter/Notch-filter for VHF 157.625 MHZ may provide 20 dB or more of rejection, depending on the Q of the resonant circuit. Mechanical design of a single high-Q quarterwave coax notchfilter using RG-213 coax are shown by DJ4XAV here http://dl4xav.sysve.de/coax.filter/coax-filter.html
. There is a limit to the width and depth of notch you can achieve by using better components, e.g. coax.
With more than one resonant circuits you can achieve about 60 dB as described by HB9AMO http://www.hb9amo.net/fmcoaxialfilter.php
or PA0NHC http://www.pa0nhc.nl/IMDfilter/EN/Cable ... rV10EN.htm
Other Notchfilter combinations are also possible e.g. consiting of two parallel tuned resonant circuits tuned for VHF 157.625 MHZ and a series tuned resonant circuit/Trap-Filter/Notch-filter for VHF 157.625 MHZ that will short VHF 157.625 MHZ to ground in the middle of both parallel tuned circuits.