Underpainting with acrylic paint can save you time, but give you the same advantage as using oils. Of course, oils and acrylics don't mix. One is oil based, the other water based. But, there are ways to combine the two. Acrylics are essentially a plastic. They have the advantage of drying quickly either thick or thin, but they do not have the blending that oils have. Acrylics can be thinned very easily using water and can be used for glazing.
Begin with a thin wash of acrylic over the gessoed canvas or board. Once it has dried which will be a few minutes, you can then draw your design with a brush and thinned acrylic. Blocking-in of the shapes can also be done with the acrylic paint. Remember to keep your paint thin and flat. Do not have thick glossy acrylic paint. The oil will not adhere properly to the acrylic paint in this case.
Now, it's time to start with the oils. Make sure that your acrylics are absolutely dry. Start with thin oil paint, then build texture. Always remember the adage "Fat over lean" or "Thick over thin". Thin paint, then thick. This lets oil paints dry properly. Golden Paints has printed some information about the guidelines of oil over acrylics.
I've used the acrylics and oils in this way before as a time saver when I have deadlines. I've also accidently grabbed a couple of acrylic paint tubes thinking they were oils. Oil and acrylics definitely don't mix. I had a mess!