When you say you have an event every 2 days except weekends, I suppose you're trying to say: Mo-We-Fr (week 1), Tu-Th (week 2), Mo-We-Fr (week 3)... is that right?
The basic structure of iCalendar (the one used by Rainlendar) is more complicated than you think. The standard takes for example this Recurrence Rule: RRULE:FREQ=YEARLY;INTERVAL=2;BYMONTH=1;BYDAY=SU;BYHOUR=8,9;BYMINUTE=30
This means: an event that happens at 8:30 and 9:30 every Sunday of January every 2 years.
Of course, I'm not saying that what you want is something impossible. In fact, your rule would be: RRULE:FREQ=DAILY;INTERVAL=2;BYDAY=MO,TU,WE,TH,FR. Or only with the interval and setting the exception rule as EXRULE:FREQ=WEEKLY;BYDAY=SA,SU. The problem is that there are thousands of combinations of the recurrence rule.
Maybe you have luck and Rainy add your rule.
As a workaround, I suppose you can split it in 2 events:
The first one, on Monday, Wednesday and Friday every 2 weeks;
the second one, on Tuesday and Thursday every 2 weeks, starting the next week of the above.