Have you tried to convert both of the packages into 2GMP form?
This way, you could deploy the unmanaged code for both packages in the same scratch org
{
"packageDirectories": [
{
"path": "force-app",
"default": true,
"package": "Baseline",
"versionName": "0.0.0",
"versionNumber": "0.0.0.NEXT",
"ancestorId": "04tBase000Line0AAA"
},
{
"path": "dependent",
"package": "Dependent",
"versionName": "0.0.0",
"versionNumber": "0.0.0.NEXT",
"dependencies": [
{
"package": "Baseline",
"versionNumber": "0.0.0.LATEST"
}
],
"default": false
}
],
"namespace": "Namespace",
"sfdcLoginUrl": "https://login.salesforce.com",
"sourceApiVersion": "52.0",
"packageAliases": {
"Baseline": "0HoBase0000Line0AAA",
"Dependent": "0HoDEPENDENT000AAA",
"[email protected]": "04tDEPENDENT000AAA",
"[email protected]": "04tBase0000LineAAA",
}
}