We propose to observe Cyg X-1, an accreting black hole, and Sco X-1, an accreting neutron star, with a 2-hour long uninterrupted exposure each with IRAC in high time resolution modes. Their rapidly variable IR components are likely to come from relativistic jets, and these data will provide important insights into how these jets are powered by the systems' accretion disks. While jets appear as a byproduct of the accretion process, there is as yet no standard model for their underlying physics. Several key issues are be solved, including the geometry of jets, their energetic power and their coupling with the disk. Multwavelength fast variability is an exciting new tool, which will provide new informations. For the first time, we will study the fast jet variability in great detail, opening a new promising window on the Physics of accretion. These explorative observations will provide a measure of the size of the emitting region, thus constraining the jet geometry. By studying the correlated variability in mid-IR and X-rays, we will put important constraints to the jet-disk connection. Finally, by comparing these results between the two sources, we will cast light on the jet energetic budget, which is thought to be different in black holes and in neutron stars.