dynamic marks on multiple lines between piano staves

[Sirius Barras]

I would like to write *between* the piano staves, on two different lines

1.p

2.f

where p and f are dynamic marks. What I should do? And by the way do you suggest a different typographical solution?

Thank you, s.

P.S. I read the manual, googled online but I found only partial solution to my problem.