I've done sums on pi networks for 10 and 14 MHz, which suggets you can do it OK.
However, the technique is design for Q of about 13 at at the geometric average
of the two frequencies, and then work backwards from the value of inductance
thus achieved to check what you get at the two frequencies in terms of working Q
when you tune that value of inductance.
With US hams being limited to 200 watts on 10MHz, there's not likely to be
anybody in the US who has done the modification (or who would admit to it if
they had!)
So you would need to change the existing 20m tap, which could be somewhat
difficult.
73
Peter G3RZP
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