Can a viable embryo develop from the fusion of two egg cells?
Yes, it is possible to make a zygote from two ovules. Other more complex scenarios are possible as well. You might want to read this cbc article. If the two ovules came from the same organism, then the baby will very likely suffer from a number of diseases due to very low heterozygosity (same mechanism that is causing inbreeding depression and problems of consanguinity).

It is likely not possible to get a zygote from two spermatozoids though because you would lack mitochondrial DNA (mtDNA). There is a little bit of mtDNA in spermatozoids but they are generally not transmitted. Through experimental setup I imagine it might be possible. Also, spermatozoids are very small and would lack many of the ressources necessary to start a zygote.