The following scripted pipeline prints out a map of the environment variables returned by by each p4 checkout run in parallel. The map in both cases contains the client from the second checkout (Client_2) as the value for P4_CLIENT.
node() { parallel ([ BranchOne: { node { def scmVars = checkout poll: false, scm: perforce(credential: "Credential", populate: syncOnly(force: false, have: true, modtime: false, parallel: [enable: true, minbytes: '0', minfiles: '0', threads: '0'], pin: "", quiet: true, revert: false), workspace: manualSpec(charset: 'none', name: 'Client_1', pinHost: false, spec: clientSpec(allwrite: false, backup: false, clobber: true, compress: false, line: 'LOCAL', locked: false, modtime: false, rmdir: true, serverID: '', streamName: "//stream", type: 'WRITABLE', view: ""))) echo "${scmVars}" } }, BranchTwo: { node { def scmVars2 = checkout poll: false, scm: perforce(credential: "Credential", populate: syncOnly(force: false, have: true, modtime: false, parallel: [enable: true, minbytes: '0', minfiles: '0', threads: '0'], pin: "", quiet: true, revert: false), workspace: manualSpec(charset: 'none', name: 'Client_2', pinHost: false, spec: clientSpec(allwrite: false, backup: false, clobber: true, compress: false, line: 'LOCAL', locked: false, modtime: false, rmdir: true, serverID: '', streamName: "//stream", type: 'WRITABLE', view: ""))) echo "${scmVars2}" } }]) }