Connecting full nodes within a LAN to speed up blockchain sync
Ok got it, it seems you need to remove the line bind=127.0.0.1:8333 from the config file, and (surprisingly) you can keep the setting onlynet=onion (you still can connect via ipv4 on the LAN). So the existing Tor node can be configured as follows:

txindex=1
debug=mempool
daemon=1
onlynet=onion
onion=127.0.0.1:9050
port=8333
listen=1
externalip=&lt;myexternaltoraddress&gt;.onion
seednode=&lt;seed1&gt;.onion
[...]
seednode=&lt;seedk&gt;.onion
banscore=10000
bantime=11


and the new local ipv4 node can be configured as follows:

txindex=1
debug=mempool
daemon=1
connect=192.168.0.xxx:8333


With these settings, the Tor node works as normal with Tor only connections (inbound and outbound) with the exception of the local ipv4 connection from the new node.