Allowing a collision

[Paul Hodges]

I have a score which has two staves, for unrelated instruments.  To reproduce the composer's notation, I occasionally need to join the stems of simultaneous notes on the two staves.

In a piano staff this is catered for using the \crossStaff command.  However, in my case this is inapplicable; so I am trying either to lengthen the stems on one staff or to draw a line joining them, using \markup.

However, whichever method I try, LilyPond prevents the extended stem/line from reaching note on the other staff, because it prevents the collision from happening.  

Is there a way to turn off the collision avoidance for this specific item in isolation?

Regards,

Paul