The entire point is that musicians deliberately adjust their tuning on fretless instruments continuously - and it is this that create the sound of an ensemble. you use a finger tip to form the note, and then you wobble - some do it better than others, but we're not talking out of tune, we're talking microtones - and delay and fixed pitch shifts do not do this. If you spend thousands on sample libraries, the quantity of samples is huge and you may have 5 different violins, 4 different violas - then same with cellos and basses - you do not always select the first one and repeat it. They need to be different, that's my point. sorry to grumble away on this, but creating the sound of multiple players needs effort. You can record string parts using less good samples or synths and few people ever bother to use the pitch bend wheel while they play. This can make them come alive. Real instruments have science behind them. you sometimes hear pitch bend on clarinets and oboes - clarinets can pitch bend, oboes can't (well, a tiny bit really) but people simulate them badly. strings have fixed open perfectly tuned notes, and then everything else will be off - just a bit. Some notes will slide to the next, others will have gaps. vibrato comes in after the note has formed - but how long? Every player will be different. Ensembles are just groups of differently sounding instruments from exotic to student, and every player does it differently. Each one listens to the others - hence why they find one eared headphones essential. However, the biggest errors come from looking at the lines played with the key editor most DAWs have. Is every single note velocity and note length different? If they are all the same, that's a mistake. if you have a slow sounding patch or sample, then they need to be played slightly ahead of the beat. A more strident sound later. As a piece travels from ppp to fff, their beat placement changes. Is a drop from F down to E two notes, or one note with a semitone pitchblende suddenly applied? Does the violin sound play below the lowest note of the real thing? Beginners really abuse this one. A viola sound goes lower, but some synths let the violin sound go lower than real life and the tone of the string section changes as the content gets lower.
All this stuff is obvious to a string player - and alien to others. To create realism takes work, and money - let alone time. There is no preset you can click in Cubase to make a solo line an ensemble. you might just as well add a chorus effect, it'll work just as well. If you want a realistic ensemble you need to be able to play keyboards and have patience. Even if you spend 900 quid on a spitfire wonderful string collection, you HAVE to play them!!!