If you are using only heart rate for your runs, you can sync your runs into your cycling profile but ensure you do not have HRDM enabled on your cycling. Then click on the run in the Planner and key in an XSS number that you feel would apply to your cycling from each run. An example would be to use 40XSS per hour and likely use Endurance focus if the run was just a regular endurance run. If the run was a high-intensity workout, you could up the rate and choose a Climber or GC Specialis focus. Again you’d have to experiment a bit with it.
If you are using running power, then set up a separate profile with separate fitness signature and take your run’s XSS and factor it down by maybe 10-20% into your Cycling profile. So if it was a 100XSS run based on your power data, use 80-90XSS into your cycling profile and stay with the same Focus/Specificity. Unless you intend on doing a long period of just running and no cycling and you want that to be properly reflected into your cycling fitness profile when you start up again, the precision isn’t that important.
Note that the fatigue from running is higher than what gets accounted for in just the XSS numbers so you might need to increase your Recovery Demands and/or adjust Freshness Feedback if you’re using the Adaptive Training Advisor. Optionally, simply following what you’re feeling and choosing appropriately is just as good. Only issue is having a well identified ramp up towards a goal like XFAI does will be difficult to create. You’ll need to go more by feel than have a predictive plan of fitness gains.
Hope this helps.