FWIW, and since you mention it as a potential workaround, RFC5545 has removed the ability to have multiple RRULE property in a single VEVENT/VTODO (https://www.rfc-editor.org/rfc/rfc5545#section-3.6.1). It was there in RFC2445.
Then, unless someone finds a very elegant and simple solution, you may be left with something that is so convoluted that most clients will barf on it (don't know how interoperability is important to you).
This leaves you with 2 choices I think:
- either use 2 different events (maybe linked with a RELATED-TO property)
- or, if you can bound the event to a certain number of instances, represent it as a set of RDATE (or RRULE + RDATE, or RRULE + EXDATE, etc...).