1. No, the ISDNLink is designed to only be paired with a single SX20. If you have two SX20s, you would need to ISDNLinks (and applicable ISDN lines).
2. What version of software are you running on your SX20 that you are having issues with? You may be running an older software release that doesn't give you the option. The current software release for an SX20 is CE9.4.0.
Wayne
Please remember to mark helpful responses and to set your question as answered if appropriate.