We propose to measure the expansion parallax of the nebulae around the binary symbiotic Miras He 2-104 {the Southern Crab} and He 2-147 by means of high resolution WFPC2/F658N images to be taken at two different epochs. Given the large outward velocities of these nebulae inferred from ground-based spectroscopy, their apparent expansion can be measured by HST imaging over timescales of only 2-3 years. For He 2-104 we have already obtained ``Epoch 1'' HST images {cf. STScI-PRC- PR99-32}. Combining the angular and radial velocity expansions will straightforwardly yield the distance to the proposed targets. Knowledge of the distance to these nebulae will provide a solid contribution to a variety of research fields such as {a} the energetics of nuclear burning in symbiotic binaries containing a Mira, {b} the pulsation mode of binary Miras, and {c} the viability of Mira symbiotics as supersoft X-ray sources and potential progenitors of Type Ia supernovae. HST high resolution imaging of the nebulae combined with ground-based spectroscopy will also address the still debated issue of the formation of bipolar nebulae by collimated outflows from evolved stars.