OK, here's what I set up. It could do with a little more refinement but the basics are there.
SID machine with a SAW wave (for simplicity).
LFO 1 PAGE - PTCH
LFO 1 DEST - 10CT
LFO TRIG - TRIG
LFO 1 WAVE - TRI
LFO 1 MULT - x64
LFO 1 SPD - 1
LFO 1 INTL - 0
LFO 1 DPTH - 32
Then LFO 2 to mod the speed of LFO 1
LFO 2 PAGE - LFO1
LFO 2 DEST - SPD
LFO 2 TRIG - ONE
LFO 2 WAVE - SAW
LFO 2 MULT - 8x
LFO 2 SPD - 48
LFO 2 INTL - 0
LFO 2 DPTH - 48
You can also add a third LFO to fade in the depth
LFO 3 PAGE - LFO1
LFO 3 DEST - DPTH
LFO 3 TRIG - ONE
LFO 3 WAVE - SAW/RAMP
LFO 3 MULT - 1x
LFO 3 SPD - 64
LFO 3 INTL - 0
LFO 3 DPTH - 80
You can get some nice stuff by playing around with LFO1 DEST
Hope that gives you some idea on how to do it.