Thanks for the feature request!
For some extent this is already possible on the Xamarin stacks, as those include both Xcode and Android SDK, but the updates are tied to Xamarin updates.
We do have plans for a separate “cross platform” stack, where update schedules are not tied to Xamarin, but we don’t have an ETA yet.
Alternative, already available solution
Btw the workaround / alternative (which has its own PROs and CONs of course) right now is to register the same repository two times, once as an iOS app (using a Mac/Xcode stack) and once as an Android one (using the Android/Linux stack).
This approach has the advantage that if you have 2 concurrencies then it will most likely finish faster than a single build which would include building both, as well as that the Linux environment is way more customizable and easier to debug due to docker support - see for example this DevCenter guide about how you can download the Linux environment and run your build in it locally, using docker.