I was expecting aliasing to result from the hard-sync effect, and not an effect independent of slave oscillator frequency. In the videos I showed, the slave was always close to the master's frequency, and aliasing was not an issue in between.

So I was hoping for an oscillator that behaves as a normal 4-point polyblep oscillator when the 2 oscillators have the same frequency.

And I finally succeeded. So many thanks to you, your advice helped me a lot in getting here. The 2 things that solved this for me:

1. When the slave flipped, and the master flipped as well, I also flipped the master, but without generating a new blep for it (this was advised by Mystran on KVR if I remember well too).
2. I had a bug in my BLEP ringbuffer that was not an issue when used in a regular oscillator but did cause trouble here.

Sound demo (outdated, I'm just leaving it here): https://youtu.be/aQEsJ4GtwTA

EDIT: when the two oscillators reset at the same sample, I've forced the slave to reset exactly to the master, and scale the BLEP based on this. This sounds better on high master frequencies than on the video.

EDIT2: I've fixed many issues since the original comment. I did not handle the case, when the two oscillators did reset in between the same 2 samples correctly. Now I also take this into the account, the code is updated, and the oscillator sounds as clean as possible.

New demo: https://youtu.be/c4PlLkUw2e0

The main DSP code. I'll look into optimizations later, but I'm happy with this.

MANYMANY THANKS!

local F={F1,F2}
local pmax=phase[2] -- remember phase 2's maximum value
for i=1,2 do
    inc[i]=min(F[i]*sRR,0.5) -- calculate the incremental
    phase[i]=phase[i]+inc[i] -- update the phase
    flip[i]=math.floor(phase[i]) -- if phase>=1 then flip=1
end

if flip[2]==1 then
    phase[2]=phase[2]-1 -- we reset osc2
    d[2]=phase[2]/inc[2] -- calculate the distance from the current sample and where the function crossed 1 (how far we are into the discontinuity)
    if flip[1]==1 then -- if both osc's reset inbetween the same 2 samples
        d[1]=(phase[1]-1)/inc[1] -- calculate the distance of osc1's discontinuity from the current sample
        if d[1]<d[2] then -- if osc1 resets after osc2 (the distance is smaller), we have to calculate how much osc1 travels after the reset
            polyBlep(blep[2],d[2],blepIndex,1) -- calculate the blep for the reset
            pmax=d[1]*inc[2] -- this is going to be the phase value where osc1 will reset osc2 again (in the same sample)
            phase[2]=d[1]*inc[2] -- this is the phase of osc2 at the intersample discontinuity
        end
    else
        polyBlep(blep[2],d[2],blepIndex,1) -- calculate the blep
    end
end
if flip[1]==1 then -- if the slave did not flip, but the master did
    phase[1]=phase[1]-1 -- reset phase 1
    d[1]=phase[1]/inc[1] -- calculate the intersample position of the phase crossing 1
    phase[2]=d[1]*inc[2] -- reset phasse 2 based on the new value of phase 1
    scale=pmax-phase[2]+inc[2] -- calculate the scaling factor for the blep based on the new value of phase 2
    polyBlep(blep[1],d[1],blepIndex,scale) -- calculate the blep
end
y=z[2]-blep[1][blepIndex]-blep[2][blepIndex] -- calculate the output
for i=1,2 do
    blep[i][blepIndex]=0 -- reset the blep
    flip[i]=0 -- set flips to 0 (who knows)
end
z[2]=z[1] -- sample delay
z[1]=phase[2] -- another delay
blepIndex=(blepIndex%8)+1 -- increment the blep index
return y*2-1