If you already use Kotlin Gradle DSL, the alternative to using it this way:
Here's my project structure
|-root
|----- app
|--------- libs // I choose to store the aar here
|-------------- my-libs-01.aar
|-------------- my-libs-02.jar
|--------- build.gradle.kts // app module gradle
|----- common-libs // another aar folder/directory
|----------------- common-libs-01.aar
|----------------- common-libs-02.jar
|----- build.gradle.kts // root gradle
My app/build.gradle.kts
- Using simple approach with 
fileTree 
// android related config above omitted...
dependencies {
    // you can do this to include everything in the both directory
    // Inside ./root/common-libs & ./root/app/libs
    implementation(fileTree(mapOf("dir" to "libs", "include" to listOf("*.jar", "*.aar"))))
    implementation(fileTree(mapOf("dir" to "../common-libs", "include" to listOf("*.jar", "*.aar"))))
}
- Using same approach like fetching from local / remote maven repository with 
flatDirs 
// android related config above omitted...
repositories {
    flatDir {
        dirs = mutableSetOf(File("libs"), File("../common-libs") 
    }
}
dependencies {
   implementation(group = "", name = "my-libs-01", ext = "aar")
   implementation(group = "", name = "my-libs-02", ext = "jar")
   implementation(group = "", name = "common-libs-01", ext = "aar")
   implementation(group = "", name = "common-libs-02", ext = "jar")
}
The group was needed, due to its mandatory (not optional/has default value)  in kotlin implementation, see below:
// Filename: ReleaseImplementationConfigurationAccessors.kt
package org.gradle.kotlin.dsl
fun DependencyHandler.`releaseImplementation`(
    group: String,
    name: String,
    version: String? = null,
    configuration: String? = null,
    classifier: String? = null,
    ext: String? = null,
    dependencyConfiguration: Action<ExternalModuleDependency>? = null
)
Disclaimer:
The difference using no.1 & flatDirs no.2 approach, I still don't know much, you might want to edit/comment to this answer.
References:
- https://stackoverflow.com/a/56828958/3763032
 
- https://github.com/gradle/gradle/issues/9272