Don't bother buying a new Phil and Teds, buy a secondhand ebay one from someone who hardly used thers, new instantly loses value, you may not use it enough to match its devaluation, and plus it will get a bit trashed when toddler learns to "step on the brake" as my DD puts it !!! plus crumbs, mildew etc, etc
We had a secondhand one all summer in the lying-down baby position, first with coccon, then enormous DS just wedged into back pocket with blankets. Today tried out DS (6 months) sitting up in good old (second-hand) Bugaboo that has been sitting neglected in shed, with 2.5 DD walking/running next to me, and it worked fine. we hitched a few bus lifts during the day, but mostly was fine, she never once asked to go in pushchair or be picked up.
Word of advice: ALWAYS carry a sling to pop youngest into, even if you do have a double buggy. Sometimes they will scream for no apparent reason, yet will be soothed in a sling so although you may look an idiot with toddler walking next to empty buggy while you carry baby and push said empty buggy, it is worth a bit of peace and quiet.
I did buy my slings new, should have followed my own advice, but actually I have used them a lot, so not too foolish there....