This going to be something of a fiddle.
Place the two clips one above the other on a Premiere Pro CS4 timeline.
Apply the Strobe effect to the top clip with the duration half of the period and the strobe set to make the layer transparent. This will cause the lower layer to appear half the time interleaved with the top layer.
Now the fiddling starts.
The period of the strobe is in seconds, tenths and hundredths, which makes it very hard to split exactly on every frame.
However, it should be possible to use File/Interpret Footage to select a frame rate for both clips which is compatible with tenths of seconds matched to the strobe frequency set in the effect so that the swaps happen as you describe.
If you are lucky and patient, you now have alternating frames on a timeline at the wrong speed.
To simplify your next step, nest this timeline (sequence) inside another sequence and use speed keyframes to get the speed changes you want.
Be aware that seriously slowing footage is unlikely to be very attractive visually. Best results when using exact divisors or multiplies (eg 1/2, 1.3, 2x, 3x etc) to avoid Premiere interpolating frames.
Have fun !