Bring salted water to boil. Prepare pasta according to package directions, al dente.
Melt butter and slowly whisk in flour and cook 1 minute, whisking constantly.
Slowly whisk in milk (don't pour in all at once, you won't get the same consistency- trust me). Cook over medium heat, whisking until mixture thickens and bubbles. Remove from heat.
Add both cheese stirring until fully melted and incorporated.
Add pasta to sauce. Continue cooking for about 1 minute. If the sauce seems to thick, you can always add a little bit of the pasta water.