Actually I found two ways how to make it work. Sometimes the mkgmap style-file code allows so many nice workarounds. :-)

a) the odd way
rcn_ref=* & rcn_ref=10    { name '${rcn_ref}' } [0x01701 resolution 22 continue]
rcn_ref=10                        { name '${rcn_ref}' } [0x01800 resolution 22]

b) the logic way (which can be used for even more than 2 symbols) - actually it can be used to make as many overlays as one would like to have (only adds a bit of code to the style-files, using an overlays file would be a bit "cleaner").
rcn_ref=11                { set mkgmap:rcn_ref=11; name '${rcn_ref}' }     [0x01701 resolution 22 continue with_actions]
mkgmap:rcn_ref=11                                        { name '${rcn_ref}' }     [0x01801 resolution 22]




Now that we know both ways we can think of how to achieve our goal while using the fewest code (assuming 0-99 only, if 0-999 it would be a bit more complex)
rcn_ref=* & rcn_ref > 90 & rcn_ref < 99 { set mkgmap:rcn_ref='${rcn_ref}'; name '${rcn_ref}' } [0x..... continue with_actions]
rcn_ref=* & rcn_ref > 80 & rcn_ref < 89 .....

We need rcn_ref=* in this case, because dropping it makes it invalid for mkgmap style-parser....