If you want to run fast, the best thing is running. If you cant run every day, biking is worlds better than doing nothing.
As others have said, be careful not to make every day on the bike be a hard day. Dont try to do the training that conpetitive cyclists do (ie, high spin days, high load/slow spin days, etc). Instead, emulate easy run days of 45 to 60m at a running cadence most days. Once a week or so, throw in some tempo equivalents or a fartlek replacement.
Then on your two running days per week, do a hard day most days: either a tempo run, long run, or intervals.
Stationary bike is great for me too, I'm in a similar situation if limited number of days I can run per week. I can bike while the kid takes a nap, lunchtime when it is too hot, or at night when I would otherwise need to run with a headlamp.