The -Filter parameter in Get-ChildItem only supports a single string/condition AFAIK. Here’s two ways to solve your problem:
You can use the -Include parameter which accepts multiple strings to match. This is slower than -Filter because it does the searching in the cmdlet, while -Filter is done on a provide-level (before the cmdlet gets the results so it can process them). However, it is easy to write and works.
#You have to specify a path to make -Include available, use .\*
Get-ChildItem .\* -Include "MyProject.Data*.dll", "EntityFramework*.dll"
You could also use -Filter to get all DLLs and then filter out the ones you want in a where-statement.
Get-ChildItem -Filter "*.dll" .\* | Where-Object { $_.Name -match '^MyProject.Data.*|^EntityFramework.*' }