How split SA staff into two staff - each now has two voices?

[MING TSANG]

Hi, lilyponders:

I try to engrave a hymn which has two staff - one for SA and one for TB.  At measure 70 to the end (measure 91)  SA staff split into two S & A and each contains two voices. TB stays on one staff till the end.

SA   measure 1 to 69 then it split into S & A measure 70 to 91. Each S & A contains two voices.

TB stays in a staff measure 1 to 91.

Emmanuel,

Ming.