Cross-staff notes using only one music expression

[Paolo Prete]

Hello,

in the following snippet, I can obtain a cross-staff tremolo:

http://lilybin.com/y4csd8/1

However, in this way, I'm forced to fill the "up" staff with invisible rests ( in this case:  s4. ) for the duration of the tremolo. Is there a way to avoid this and use only one music _expression_ for the cross-staff notes?

Thanks