I tried a similar test on my Pio 640 and got "mixed" results. My first test seemed to show the Pio would delay rec 2nd event till I stopped chase playing. a 2nd test indicated otherwise... not sure on the Pio yet.
Anyway, I did another test on the 3575 of FOUR 3-MIN. BTB timer programs on different channels, and the recording proceeded as stated above... 3-sec buffer-write and switch to next channel. This time, however, I waited to Chase Play while the #2 program was recording, to see if #1 or #2 would play. It chased #2 program. In trying to see if I could get back to #1 program, I went to Title menu but it bounced back to the rec of #2 program. Turns out the 3-min. test programs were too short... there is some 3-4 min. thing on chase play that prevents FF and some other ops during the last 3-4 minutes or so.
So then I rec four 20-MIN. BTB timer programs and it worked as expected: played thru to #2 while chasing #1, with 3-sec HDD-buffer-write while it switched channels.
When I waited till the #2, #3 and #4 programs started recording, pressing PLAY chase played whatever recording was in progress.
I was able to use the Title menu and the INFO menu to Play any of the titles already recorded, and I could also chase play whatever title was still recording thru either of those menus. The INFO menu plays titles from frame one, regardless of their Resume position.
These BTB tests also showed that, if you prefer separate titles on the HDD, you can set up programs BTB on the same channel rather than one longer double- or triple-show that you then need to Divide to get separate titles... you essentially only lose the first 3-sec of succeeding programs.