A search for "hamippoi" on the SoA forums turned up a couple things of relevance:
Livius 48.56.2, on the battle of Kallinikos/Callinicus in 171 BC, wrote:Cotys [sc. a Thracian king allied with Macedon] commanded the left wing with the whole of his native troops, the light infantry being disposed between the ranks of the cavalry. On the right were the Macedonian cavalry, the Cretans being intermixed with them in the same way. ... The [Roman] right wing was commanded by the consul's brother Caius, and comprised the whole of the Italian cavalry with the velites interspersed among them.
There are further contingents listed on the Roman side consisting of both cavalry and light infantry, but we're not explicitly told if they're intermixed.
Caesar, Civil War III.84, on Pharsalus 48 BC, wrote:His [sc. Caesar's] cavalry being much inferior to the enemy's in number, he followed the method already mentioned; of singling out the strongest and nimblest of his foot-soldiers, and accustoming them to fight intermixed with the horse; in which way of combat they were become very expert by daily practice. This disposition, joined to constant exercise, so emboldened his cavalry, that though but a thousand in number, they would upon occasion sustain the charge of Pompey's seven thousand, even in an open plain, and appear not greatly dismayed at their multitude: nay, they actually got the better in a skirmish that happened between them, and killed Aegus the Allobrogian, one of the two brothers who deserted to Pompey, with several others of his party.
There are also plenty of mentions of light infantry of various nationalities supporting cavalry, but without it being specified if they do so as part of a single formation or if they're simply deployed alongside.
Finally, the late antique lexicographer Hesychius, presumably specifically to mess with wargamers, gives hamippoi
as an alternative designation of dimakhai
), an obscure troop type supposedly introduced by Alexander, consisting of mounted men capable of fighting both mounted and on foot (the word means something like "double-fighters"). Apart from a few mentions in the Alexander histories - never in the context of pitched battle - the word also turns up in Diodorus to describe some Spanish horsemen.