Run the following test:

import static jdk.incubator.http.HttpClient.Version.HTTP_2;

import java.net.URI;
import java.nio.file.Paths;
import java.util.Arrays;
import java.util.Enumeration;
import java.util.Set;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ConcurrentSkipListSet;
import java.math.BigInteger;
import javax.net.ssl.SSLSessionContext;

import org.testng.annotations.DataProvider;
import org.testng.annotations.Test;

import jdk.incubator.http.HttpClient;
import jdk.incubator.http.HttpClient.Version;
import jdk.incubator.http.HttpRequest;
import jdk.incubator.http.HttpResponse;

/*
 * @test
 * @compile HttpTestUtils.java
 * @modules jdk.incubator.httpclient
 * @run testng/othervm TestConnectionReuse
 */
public class TestConnectionReuse {
    private static HttpClient getClient(Version v) {
        return HttpTestUtils.clientBuilder().version(v).build();
    }
    
    @DataProvider(name = "uri")
    public Object[][] uri() {
        return new Object[][] {
                {new String[] { "https://<host>/yc/data/d31.data",
                  "https://<host>/yc/data/d32.data",
                  "https://<host>/yc/data/d33.data",
                  "https://<host>/yc/data/d34.data",
                  "https://<host>/yc/data/d35.data"
                }}
        };
    }
    
    @Test(dataProvider = "uri")
    public void testAllStrart(String[] uris) throws Exception {
        HttpClient client = getClient(HTTP_2);

        CompletableFuture[] results = new CompletableFuture[uris.length];
        Set<byte[]> sessionSet = new ConcurrentSkipListSet<>((byte[] e1, byte[] e2) -> new BigInteger​(e1).compareTo(new BigInteger​(e2)));
        
        for (int i = 0; i < uris.length; i++)
            results[i] = client.sendAsync(
                    HttpRequest.newBuilder(new URI(uris[i])).GET().build(),
                    (statusCode, responseHeaders) -> {
                        SSLSessionContext ctx = client.sslContext().getClientSessionContext();
                        Enumeration<byte[]> ids = ctx.getIds();
                        while (ids.hasMoreElements()) {
                            byte[] id = ids.nextElement();
                            sessionSet.add(Arrays.copyOf(id, id.length));
                        }
                        return HttpResponse.BodySubscriber.asByteArray();
                    });

        CompletableFuture.allOf(results).join();
        
        System.out.println("session: ");
        for (byte[] session : sessionSet)
            System.out.println(Arrays.toString(session));
    }
    
    @Test(dataProvider = "uri")
    public void testOneLead(String[] uris) throws Exception {
        HttpClient client = getClient(HTTP_2);

        CompletableFuture[] results = new CompletableFuture[uris.length];
        Set<byte[]> sessionSet = new ConcurrentSkipListSet<>((byte[] e1, byte[] e2) -> new BigInteger​(e1).compareTo(new BigInteger​(e2)));
        
        results[0] = client.sendAsync(
                HttpRequest.newBuilder(new URI(uris[0])).GET().build(),
                (statusCode, responseHeaders) -> {
                    SSLSessionContext ctx = client.sslContext().getClientSessionContext();
                    Enumeration<byte[]> ids = ctx.getIds();
                    while (ids.hasMoreElements()) {
                        byte[] id = ids.nextElement();
                        sessionSet.add(Arrays.copyOf(id, id.length));
                    }
                    return HttpResponse.BodySubscriber.asByteArray();
                });
        results[0].join();
        
        for (int i = 1; i < uris.length; i++)
            results[i] = client.sendAsync(
                    HttpRequest.newBuilder(new URI(uris[i])).GET().build(),
                    (statusCode, responseHeaders) -> {
                        SSLSessionContext ctx = client.sslContext().getClientSessionContext();
                        Enumeration<byte[]> ids = ctx.getIds();
                        while (ids.hasMoreElements()) {
                            byte[] id = ids.nextElement();
                            sessionSet.add(Arrays.copyOf(id, id.length));
                        }
                        return HttpResponse.BodySubscriber.asByteArray();
                    });

        CompletableFuture.allOf(results).join();
        
        System.out.println("session: ");
        for (byte[] session : sessionSet)
            System.out.println(Arrays.toString(session));
    }
}

It shows 5 sessions in testAllStrart, 1 new session in testOneLead(the first 5 sessions are created in testAllStrart).

I also tested h2c, but I have no means to show by program, however, I check the network traffics, it also doesn't reuse one tcp connnection in testAllStrart.