(a) Nafi' narrates that Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) would offer his voluntary prayer in the same place where he had offered the obligatory prayer.
(b) Perhaps he would speak between the two prayers, or turn aside or the like, or make a separation by some act that is permissible.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3053